Cellular Radio systems are currently in widespread use throughout the world providing telecommunications to mobile users. In order to meet capacity demand, within the available frequency band allocation, cellular radio systems divide a geographic area to be covered into cells. A base station having one or more antennas communicates with the mobile stations enabling them to communicate with other mobile stations or to subscribers of other communications networks. The available communication channels are divided between the cells such that the same group of channels are reused by certain cells. The distance between the reused cells is planned such that co-channel interference is maintained at a tolerable level.
When a new cellular radio system is initially deployed, operators are often interested in maximising the uplink (mobile station to base station) and downlink (base station to mobile station) range. The range in many systems are uplink limited due to the relatively low transmitted power levels of hand portable mobile stations. Any increase in range means that less cells are required to cover a given geographical area, hence reducing the number of base stations and associated infrastructure costs. Although, concomitant with the reduction in the number of base stations, is a requirement for an increase in the call handling capacity of each base station.
When a cellular radio system is mature the capacity demand can often increase, especially in cities, to a point where more smaller size cells are needed in order to meet the required capacity per unit area. The process used to create these smaller cells is known as cell splitting. Any technique which can provide additional capacity without the need for cell-splitting will again reduce the number of base station sites and associated infrastructure costs.
The antenna used at the base station site can potentially make significant improvements to the range and capacity of a cellular radio system. The ideal base station antenna pattern is a beam of narrow angular width as shown in FIG. 1a. The narrow beam is directed at the wanted mobile, is narrow in both the azimuth and elevation planes, and tracks the mobiles movements. When compared to an omni-directional antenna, such a beam will have the dual benefits of having high gain, leading to increased range in thermal noise limited initial deployments, and rejecting interference from co-channel reuse cells allowing higher capacity without cell splitting in mature deployments. The narrow beam reduces interference in a balanced manner on the uplink and downlink. On the uplink the base station receiver is protected from interference generated by mobile station transmitters in the co-channel reuse cells, FIG. 1b. On the downlink the mobile is unlikely to be in the beams of the base station transmitters in the co-channel reuse cells. The extent of the advantage of a narrow beam antenna over an omni-directional antenna is a function of the beamwidth: The narrower the beamwidth, the greater the advantage, but this must be traded off against the increased size and complexity of the antenna. When the narrow beams are contrasted with an omni-directional antenna, it can easily be realised that the narrow beams create high quality transmission paths with minimal interference.
Some of the potential benefits of narrow beam antennas for cellular radio have been recognised in the literature, see for example "A Spectrum Efficient Cellular Base Station Antenna Architecture", S C Swales and M A Beach, Personal & Mobile Radio Communications Conference, Warwick, 1991 and "Proposed Advanced Base Station Antennas for Future Cellular Mobile Radio Systems", W S Davies, R J Long and E Vinnal, Australian Telecomms Research, Vol. 22, No. 1, pp 53-60. Within current systems the manner in which directive antennas are used allows relatively small benefits to be obtained. The use of directive antennas in current cellular radio systems is based on the principle of sectorisation as illustrated in FIG. 2.
The capacity of a cellular system is in general limited by the amount of interference received from other cells. The carrier to interference ratio, C/I, performance of the system determines the frequency re-use strategy, by which the same frequency channels are used in neighbouring cells. The use of narrower beams produces an increase in the carrier to interference performance of the system, allowing a tighter frequency re-use pattern to be used, which in turn provides increased traffic capacity. Conventional cell sites are either omnidirectional, tri-sectored or hex-sectored, with each beam spanning 360.degree., 120.degree. and 60.degree. sectors respectively.
The main sources of interference in a cellular system come from the so called first tier reuse cells. An omni-directional base station antenna will receive interference from all six first tier reuse cells (FIG. 2a). If an antenna with nominally 120.degree. beamwidth is used, corresponding to a tri-sectored configuration, interference will be received from only two first tier reuse cells (FIG. 2b). If an antenna with 60.degree. beamwidth is used, corresponding to a hex-sectored configuration, interference will be received from only one of the first tier cells (FIG. 2c). In sectored cells the cellular radio transceivers at the base station are only connected to one sector (or antenna) and cannot be used in other sectors within the same cell.
The sectored approach to the use of directive antennas has reached its useful limit at 60.degree. beamwidth and can go no further. There are two key disadvantages of the approach: firstly, the cellular radio transceivers are dedicated to particular sectors which leads to significant levels of trunking inefficiency. In practice this means that many more transceivers are needed at the base station site than for an omni-directional cell of the same capacity; and, secondly, each sector is treated by the cellular radio network (i.e. the base station controller and mobile switches) as a separate cell. This means that as the mobile moves between sectors, a considerable interaction is required between the base station and the network to hand off the call between sectors of the same base station. This interaction, comprising signalling and processing at the base station controller and switch represents a high overhead on the network and reduces capacity.
An alternative method of improving system traffic capacity is to provide downlink beam-forming. The use of downlink beam-forming is proposed for systems such as Digital AMPS (D-AMPS). D-AMPS, also referred to as IS-54, IS-136 and TDMA-3, is the North American digital TDMA mobile telephony standard that is compatible with the AMPS analogue mobile standard. The description shall hereinafter refer to the D-AMPS as an example of a downlink beam-forming system.
A downlink beam-former typically consists of a number of narrow beams formed by either multiple narrow beamwidth antennas or an array of elements in conjunction with a fixed beam-former. Signal processing techniques can then be used to select an appropriate beam directed towards the subscriber. Alternatively, a fully adaptive beam-forming array may be employed, which allows a narrow beam to be steered towards an individual subscriber, and provides the ability to null interferers.
The D-AMPS radio standard includes a constraint on the downlink which precludes the use of the latter option of individually steered beams for each subscriber. The standard dictates that on the downlink the base station must maintain a constant level transmission during the full duration of the frame on any particular bearer channel whenever at least one mobile has been assigned to that bearer. Each traffic channel contains three time slots, and this constraint means that it is not possible to devise a scheme which allows beams to be dynamically moved from one mobile direction to another from one time slot to the next. Thus a downlink beam-former for D-AMPS has to use a fixed beam approach, where the optimum beam is deduced from uplink measurements of signal quality.
The use of a common orthogonal array aperture allows a set of narrow beams to be formed that cover the sector. However, as the beams are orthogonal, the power in each beam will be -4 dB down at the crossover points. This cusping loss leads to a decrease in the mean C/I performance of the system.
An additional traffic capacity loss is realised as a result of the trunking loss caused by tighter sectorisation. For a fixed frequency assignment scheme, where the available carriers are partitioned in a permanent manner across the beam set, the Erlang capacity of the system will decrease as the number of beams per sector increases, assuming the number of traffic channels is constant.